Pore-scale investigation of residual oil distributions and formation mechanisms at the extra-high water-cut stage

Abstract

Water flooding has been widely applied in the oilfield development as a secondary recovery method due to its effectiveness and economic feasibility. After several decades of production, numerous oil fields are normally entering into the extra-high water-cut stage (water cut higher than 90%). The extra-high water-cut stage takes an extremely important role in the whole development process since a large amount of residual oil remains trapped in reservoirs (approximately 50% to 60% of initial oil in place) at this stage. The extensively continuous distribution of subsurface oil at the low water-cut stage has changed greatly after the water cut of mature oilfields becomes over 90%. The residual oil distributes mainly in a scattered state at the extra-high water-cut stage, while water flows in a continuous state. Oilfields with high water cut are normally suffering from poor flooding efficiency, to improve water flooding efficiency, it is necessary to understand distributions and formation mechanisms of residual oil at the extra-high water-cut stage. In this study, a pore-scale simulation model is developed to exploit the potential of residual oil in a porous media. A direct numerical simulation method is employed to simulate fluid flow in a porous media, the position of interface between water and oil is determined by the phase field method. The capacity and accuracy of the model is validated by a classical benchmark: a layered two-phase flow with a variable viscosity ratio. The formation processes of residual oil and mechanisms behind this are investigated in terms of mechanics. The results show that the residual oil in porous media at the extra-high water-cut stage can be classified as five types, namely, isolated oil droplet, residual oil in pore throats, cluster residual oil, oil film and residual oil in dead end. Rock configuration, wettability and capillary pressure play important roles in the formation of residual oil. The formation of isolated oil droplet is dominated by rock configuration, capillary pressure and wettability. Capillary pressure and wettability are key factors for the formation of residual oil in pore throats. Rock configuration and capillary pressure are responsible for the formation of cluster residual oil. Wettability is the dominant factor for the formation of oil film. The relationship between cumulative water production and cumulative oil production is consistent at the pore scale and Darcy scale. Relative permeability curve is obtained based on the pore-scale simulation and an upward turning is observed after the water cut reaches 98%. Increasing injection velocity and injection of surfactant can both displace cluster residual oil, residual oil in pore throats and isolated oil droplets, which leads to the increase in oil recovery at the extra-high water-cut stage. The decline in cluster residual oil is the main contributor to rise in oil recovery. Times of increasing injection velocity have an important pact on the field development. This study is only conducted in a two dimensional porous media, and more factors (e.g., gravity) should be incorporated into our model to investigate the formation process of residual oil in the three dimensional porous media.